The present invention relates generally to the control of AC electric motors. More particularly, the invention relates to a method and apparatus for variable frequency field oriented control of an electric motor utilizing a frequency generating technique to provide improved control and to eliminate the need for speed sensing.
Vector, or field oriented control techniques are generally known for variable speed control of industrial AC motors. In general, such techniques attempt to determine and apply the proper driving signals to the motor to maintain the orientation of "q-axis" rotor flux to zero. In one known technique, termed "indirect field oriented control" rotor flux is identified by analysis of feedback signals representative of rotor shaft position and by estimating slip in the motor. Assuming the slip estimate is correct, this analysis permits resolution of the stator current into a torque producing or "q-axis" component and a flux producing, or "d-axis" component. By knowing and using these components of stator current, the motor drive can properly control the amount of current applied to the motor so as to respond to changing load requirements, while maintaining proper vector orientation (i.e. q-axis rotor flux of zero). These techniques generally provide excellent dynamic torque response and accurate steady state torque as compared to other approaches, such as slip controlled drives.
A difficulty in known field oriented motor controllers arises from thermal variation of motor parameters. When not properly accounted for, such variations can substantially degrade the performance of the controller, resulting in a slip controlled drive. Various solutions have been proposed to account for thermal variations to achieve field oriented control. In one known technique, motor voltage is sensed and used as a basis for adapting for thermal variation of rotor resistance, and the d-axis component of motor voltage is used to identify the slip gain necessary to orient the q-axis rotor flux to zero. This value of slip gain is then multiplied by the torque current command to provide the desired slip frequency. The stator electrical frequency is determined using the resultant slip frequency value. A rotor shaft encoder provides feedback of rotor position and speed, and the rotor electrical frequency can be calculated based upon this speed and the rated motor frequency and number of pole pairs. Finally, the stator electrical frequency can be determined from the rotor electrical frequency and the slip frequency. The resulting stator electrical frequency is then used to control stator current so as to maintain the desired orientation of the q-axis component of rotor flux and achieve field oriented control. An example of a field oriented motor controller of this type is described in U.S. Pat. No. 5,032,771 issued on Jul. 16, 1991 to Kerkman et al., and hereby incorporated herein by reference.
While such controllers do achieve superior field oriented control, they are not without drawbacks. For example, because sensed values of rotor position and speed are used to determine rotor electrical frequency, feedback signals must be generated by encoders or similar feedback devices and continuously analyzed. While attempts have been made to provide sensorless field oriented control, many have resulted in drives having poor dynamic performance. Moreover, certain known control implementations comprise inaccurate or low bandwidth current controllers, and inadequately compensate for second order effects such as power switching device characteristics, and thus do not achieve field oriented control.
There is a need, therefore, for an improved controller for driving AC motors that is capable of providing sensorless field oriented control. In particular, there is a need for a motor controller that is capable of determining stator electrical frequency directly from electrical signals available to the controller in the synchronous reference frame without resort to rotor speed feedback signals. Furthermore, there is a need for a field oriented motor controller with improved bandwidth that is less susceptible than existing controllers to thermal variations in motor or circuitry parameters.